Optical waveguide laser

ABSTRACT

A laser includes a soliton supporting waveguide of SiO 2  --Al 2  O 3  --P 2  O 5  with an erbium doping level of 1100 ppm, the fibre having a core radius of 2.5 μm and a core-cladding refractive index difference of 0.015 operated such that the solitons propagating in the waveguide have a soliton period greater than the amplification period of the laser.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical waveguide lasers and in particular tosoliton lasers.

2. Related Art

Ultra-fast light pulse sources will be key components in future high bitrate telecommunications systems and soliton pulse sources in particularare recognised as being valuable for long distance high bit ratesystems. A current requirement is for the generation of solitons withpicosecond optical pulses and sufficient peak powers for solitonpropagation in the 1.55 μm silica optical fibre transmission window.

Although soliton pulses are robust to quite large effects of both chirpand phase noise, the evolution of such pulses inevitably involves theshedding of radiation, leaving behind a less energetic solitonsuperposed on a continually spreading background component. Thisdispersive radiation can give rise to interaction between soliton bitsand thereby limit the information capacity of the system. The idealsoliton source, therefore, is one providing picosecond duration pulseswith temporal, spectral and power characteristics compatible with thoseof fundamental soliton pulses in the transmission fibre.

One method of generating picosecond pulses is discussed in an articleentitled "Mode-locked erbium-doped fibre laser with soliton pulseshaping" by J. D. Kafka, T. Baer and D. W. Hall, Optics Letters 14, No.22 (November 1989) pp 1269-1271. A mode-locked erbium doped fibre laseris in the form of a ring laser incorporating a 70 m long erbium-dopedfibre as the gain fibre, an integrated modulator and a 2 km length oftelecommunications fibre. The telecommunications fibre provides a degreeof soliton pulse shaping to the pulse produced by the laser.

BRIEF SUMMARY OF THE INVENTION

According to the present invention a laser including a solitonsupporting waveguide doped with a material capable of providing opticalgain is operated such that the solitons propagating in the waveguidehave a soliton period greater than the amplification period of thelaser.

The pulse energy in the pulse has to be sufficient to allow the pulse tocancel out the effects of anomolous group delay dispersion (which may bepositive or negative) through the non-linearity of the waveguide, i.e.the change in refractive index of the waveguide with optical intensity.This is the basic mechanism of soliton formation.

The applicants have determined that even through the pulses may undergolarge excursions in peak power, in a laser with mirrors of reflectivityof 100% and 4%, for example, there is a stable solution of a solitonnature when the laser is operated according to the present invention.

Preferably the laser is operated such that there are at most five pulsespropagating in the laser at any given time and with a pulse repetitionrate such that the pulses are fundamentally mode locked.

The value of the pulse energy, which can be adjusted for a given opticalwaveguide laser by adjusting the pump power, is not critical. As will bediscussed later a low level pedestal component is introduced if theenergy is too high.

The laser is preferably arranged as a ring laser. Conveniently, thelaser is a mode-locked laser to form initial pulses which then becometrue soliton pulses. The method of seeding the pulses is not anessential element of the present invention so other methods, includingself seeding may be used.

The invention is applicable generally to optical waveguides, where by"optical" is meant that part of the electromagnetic spectrum which isgenerally known as the visible region together with those parts of theinfra-red and ultraviolet regions at each end of the visible regionwhich are capable of being transmitted by dielectric optical waveguidessuch as optical fibres.

The invention is of particular application to long distance opticalcommunications systems which generally use the 1.5 μm transmissionwindow of silica optical fibres. Preferably, therefore, the opticalwaveguide comprises an erbium doped optical fibre which has a highlyefficient operation over the 1.52 μm to 1.58 μm wavelength range and isreadily coupled to an optical fibre network.

It will be appreciated that other host and dopants may be employed foruse within different transmission windows of other networks or for thegeneration of soliton pulses for laboratory experiments.

Semiconductor diode pumping of the erbium fibre laser at 1480 nm or 980nm may be a possibility along with pumping by a diode pumped frequencydoubled YAG laser.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment of the present invention will now be described by way ofexample only with reference to the accompanying drawings of which

FIG. 1 is a schematic diagram of an erbium fibre soliton laser accordingto the present invention;

FIGS. 2 and 3 are graphs of the launched pump power and output power ofthe laser of FIG. 1;

FIGS. 4 to 6 are oscillographs of the autocorrelation traces of theoutput of the laser of FIG. 1 at different operating conditions;

FIG. 7 is a graph of P_(peak) (W) of the short pulse component as afunction of 1/t² (ps⁻²);

FIG. 8 is a graph of the energy profile of the soliton pulses for anamplification period for 14 dB output coupling normalised to the inputenergy;

FIG. 9 is a graph of the change in pulse area, δ S, for an N=1 solitonafter a single amplification period as a function of L/Z₀ for (a) 3 dB,(b) 6 dB and (c) 14 dB output coupling;

FIGS. 10(a) and 10(b) are graphs of the results of a numericalsimulation for L/Z₀ =0.026 propagating over a distance of 20 km (1000amplification periods) and L/Z₀ =1.93 over 1 km respectively;

FIG. 11 is a schematic drawing of a fibre ring laser embodying thepresent invention; and

FIGS. 12(a) and 12(b) are graphs of autocorrelation traces of outputs ofthe laser of FIG. 11.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1 a soliton laser comprises a 10 m length of SiO₂--Al₂ O₃ --P₂ O₅ optical fibre host 2 with an Er³⁺ doping level of 1100ppm having a core radius of 2.5 μm and a core-cladding refractive indexdifference of 0.015. A dichroic beamsplitter 4 permits efficientcoupling of pump radiation at 532 nm from the frequency doubled outputof a mode-locked cw Nd:YAG laser 6 via an uncoated microscope objective8 adjacent an end 10 of the fibre 2 and high (˜90%) throughput of 1.55μm erbium laser output.

The end 10 of the fibre 2 was polished to form an approximately 4%output reflector while a nominally 100% reflecting mirror 12 completedthe laser resonator.

A further microscope objective 14 adjacent an end 16 of the fibre 2,optimised for transmission at 1.55 μm, was used to collimate and directlight exiting the fibre at the end 16 to a 1 mm thick silicon filter 18and a lithium niobate mode-locker 20 positioned close to the mirror 12.The filter 18 blocks residual pump light to prevent photorefractivedamage to the mode-locker 20.

The mirror 12 could be replaced with a grating to provide a bandwidthrestriction an/r tunability. Reflections from the end 16 of the fibre 2were effectively suppressed by making it a polished, 5° angle face.Further suppression was achieved by placing a silica block 17 close tothe fibre end 16 with a blob of index-matching gel 19 bridging theblock-fibre interface. Fine adjustment of the laser cavity length wasaccomplished by mounting the mirror 12 on a precision translation stage22.

The mode-locker 20 comprised a Brewster angled 3×3×10 mm lithium niobateslab mounted within a resonant LC circuit tuned to 420 MHz and used tomode-lock the laser by the phase modulation scheme discussed in the textbook "LASERS" by A. E. Siegman University Science Books ISBN 0-935-702-11-5 which has been employed with Nd³⁺ fibre laser to generate pulsesas short as 20 ps.

Fibre polarisation controllers 24 were included in order to optimise thepolarisation of the light incident on the modulator.

The output pulse durations of the mode-locked laser were recorded usingthe standard non-collinear, second harmonic auto-correlation technique.Spectral data were recorded using a scanning grating spectrometer (notshown). Output powers were measured with a Scientech 362 power meter andcorrected for the known transmission characteristics of the combinationof the lens 8 and the beamsplitter 4.

Since the upper state life-time of the Er³⁺ ion is long (9.8 ms), thepump pulse train (˜50 ps in duration at a repetition frequency of 76MHz) is integrated and essentially looks like a CW pump.

The performance of the 532 nm pumped CW erbium laser is illustrated ingraphs of FIGS. 2 and 3. For the experiments in which the data shown inthese two graphs were obtained, a 1200 line/mm gold coated gratingreplaced the mirror 12 and modelocker 20 arrangement of FIG. 1. Powersin excess of 100 mW were available over the 1.52-1.58 μm range for anestimated launched pump power of 600 mW as shown in FIG. 2. At the peakof the tuning curve, about 1.56 μm, an output of about 140 mW wasobtained as shown in FIG. 3. Assuming a 70% reflectivity for thegrating, the maximum expected slope efficiency is about 26%. This is ingood agreement with the 24% slope efficiency inferred from FIG. 2 and isa consequence of the low excited state absorption at 532 nm.

The mode-locked operation of the erbium fibre laser of FIG. 1 isdepicted in FIGS. 4 to 6, where there are shown autocorrelation tracesof the laser output under various conditions. Since the round-triperbium fibre length was 20 m, there are ˜40 laser pulses in the cavityat any one time. By incorporating suitable bandwidth restrictingelements inside the laser resonator, for example a birefringent tunerplate, pulse durations ranging from 40 ps down to 15 ps (assuming asech² pulse shape) were produced with time-bandwidth products ΔtΔνof0.5-1.0 respectively. The higher products for the shorter pulses areindicative of the increased role of nonlinear phenomena in the pulseformation process.

FIG. 4 shows an autocorrelation trace of 17.3 ps pulses (ΔtΔν) obtainedwith a 0.5 mm thick quartz birefringent tuner plate. Average outputpowers of the laser were in keeping with the performance depicted inFIG. 2. As expected for phase modulation mode-locking, two sets of 420MHz pulse trains were observed corresponding to either extremum of thephase modulation. Either set can be selected through etalon tuning (viathe filter 18) and fine adjustment of the focus of lens 14. In addition,these adjustments facilitated some control of the laser bandwidth andhence the pulse-width.

When the laser bandwidth restriction was removed, further pulsecompression was observed. Low output pulses (less than ˜5 W peak) withdurations in the 3-5 ps range were recorded with time-bandwidth productsof ˜0.4. By increasing the pump power, however, pulse durations if 2-3ps were generated with time-bandwidth products in the range 0.3-0.35.

FIG. 5 shows an autocorrelation of 2.9 ps pulses recorded at an average(peak) power of 8.5 mW (6.9 W). A sech² pulse shape is an excellent fitto the autocorrelation and is also consistent with the measuredΔtΔv=0.3. These pedestal-free pulses clearly have the appropriatetemporal and spectral characteristics for fundamental solitons. Previouswork on femto-second amplification in erbium fibres with similar dopantlevels and core geometrics to the above fibre is consistant with a lowpositive group delay dispersion. If we take D=5 ps/nm/km and A_(eff) =30μm², we calculate a final soliton power P₁ =0.5 W for a pulse width of 3ps.

Although this is significantly lower than the measured output, thediscrepancy is not surprising when we consider that the laser outputcorresponds to the peak of the energy changes occurring within thedistributed amplification.

At higher output powers, although a significant compression of thepulse-width was observed, a low level pedestal component was clearlyevident. FIG. 6 shows the output of the laser at 48 mW average power.From both autocorrelation and spectral measurements, ˜53% of the outputenergy was estimated to be in the pedestal, which implies a peak powerof 44 W for the short, 1.2 ps component. It is noteworthy that even inthe pedestal region, the autocorrelation and spectral measurements ofthe short component were still in accordance with a sech² pulse shape.

FIG. 7 is a graph of the data recorded over a range of output powers asthe pump power was increased into the pedestal region. It indicated thatthe peak power of the short component scales in direct proportion to1/t². These results add further weight to the notion of solitonformation.

It is informative to compare these results to the predictions of thestandard Kuizenga and Siegman FM mode-locking model for a homogenouslybroadened laser medium. For the case of zero frequency detuning, typicalvalues of the saturated round-trip gain coefficient (G₃ ˜1.6), andmodulation depth (Δ_(m) ˜0.1), lead to a Gaussian pulse width dependingprimarily on 1/(f_(m) Δf₃)^(1/2) where f_(m) is the modulation frequencyand Δf₃ is the effective gain bandwidth. If we take f_(m) =420 MHz2^(1/2) 0.44 and Δf₃ =1 THz, we therefore expect a chirped (ΔtΔv=x0.44)pulse with a duration of ˜50 ps. By detuning the drive frequency, themode-locked pulses can be compressed (to ˜35 ps) and thereby dechirpedsuch that ΔtΔv=0.44. It is clear that the model incorrectly predictsboth the pulse shapes and durations observed in our experiments.

We believe that in order to fully explain our observations, theinterplay between fibre nonlinearity and dispersion must be invoked. Inthe case where the group delay dispersion is positive, then soliton-likecompression to picosecond duration sech² pulse shapes would be expectedto follow. Indeed, femtosecond pulse amplification in erbium fibre withdopant levels and core geometries similar to our sample, are consistentwith a low, positive group delay dispersion (λ₀ ˜1.5 μm).

In the model presented here, we have numerically solved the NonlinearSchrodinger Equation (NLSE) with a periodically varying pulse energy. Asa crude approximation to the laser configuration, we employed adistributed gain (G˜14 dB) which exactly cancelled the large lump lossoutput coupling (96% transmission) for the resonator. An amplificationperiod, L, of 20 m was chosen, i.e. the round-trip fibre length of theresonator. We also take a low, positive group delay dispersion D=5ps/nm/km, in keeping with the dispersion-shifted properties of theerbium fibre. A simple uniform amplitude gain coefficient, G, is assumedthroughout the amplification period such that the pulse energy, E=E_(in)e^(2GZ), where E_(in) is the input energy, and Z is the distance a pulsehas propagated along the fibre.

FIG. 8 illustrates the pulse energy profile assumed in the computersimulations. The dashed line represents the average energy, E_(3v). Inaddition, owing to the long life-time 0.10 ms) and low gaincross-section (10⁻²⁵ cm²) of the erbium ions, the effects of gainsaturation during the pulse are neglected. (For picosecond pulses thecalculated saturation energy is .sub.˜ 20 μJ, i.e. six orders ofmagnitude greater than the typical output pulse energies).

The key result of the numerical simulations is as follows: Stablefundamental soliton propagation is achieved in the limit of a "long"soliton period (i.e. Z₀ >>L) when the average energy in theamplification period, E_(av), is set equal to that of the fundamentalsoliton energy, E₁. The input pulse energy is therefore set by thefollowing equation

    E.sub.in =E.sub.1.2GL/(e.sup.2GL -1)                       (1)

For example, in the .sub.˜ 14 dB loss case, (i.e. exp^(2GL) =25), thenE_(in) =0.134E₁.

FIGS. 9, 10(a) and 10(b) illustrate the essential results of thesimulations. FIG. 9 shows the changes in pulse area, S (normalised tounity), for the fundamental soliton (with E_(in) calculated as above) asa function of L/Z₀ after a single amplification period. The pulse area(defined as the time integral of the absolute value of the pulseamplitude) is commonly utilised as a sensitive indication of thedistortion from a true soliton. Clearly, in the "long" Z₀ limit, thecomputed pulse distortions are negligible. This is also apparent in FIG.10(a), where a simulation over a total distance of 20 km (.sub.˜ 26soliton periods) is depicted for L/Z₀ =0.026 and 14 dB output loss(t.sub.˜ 3 ps). In FIG. 9 we also show the computed pulse distortionsfor 3 dB and 6 dB output losses. As expected, by reducing the magnitudeof the energy deviations (and thereby approaching the ideal losslesslimit) pulse distortions can be significantly reduced.

It is important to note that operation in the long soliton limit ispreferred but that the operation regime of the present invention extendsto Z₀ >L as discussed below. It is informative to calculate the solitonparameters associated with pulses generated by the laser. For a 3 pspulse, a dispersion of 5 ps/nm/km and an effective area, A_(eff) =30μm², we obtain Z₀ =705 m and a fundamental soliton power, P₁ ˜0.5 W.Clearly, the long soliton period criterion is easily satisfied with L/Z₀˜0.03. The calculated value of P₁, however, is significantly less thanthe measured output power of 5 W. This can be understood in terms ofFIG. 8. We must remember that the important soliton parameter is thepath average value, E_(av), represented by the dashed line in FIG. 8. Inthe case of our simplified energy model, the energy at the peak of theprofile (96% of which is coupled out) is approximately 3×E_(av). Inreality, a more accurate modelling of the energy profile, including, forexample, the effects of pump absorption and additional intracavity loss,would be expected to increase the appropriate multiplication factor.

A further characteristic of the laser which can be understood in termsof our model is the P_(peak) vs. 1/t² behaviour depicted in FIG. 7. Ifthe pulse area, S is preserved during propagation, then E scales as D/tor equivalently P_(peak) is proportional to D/t². In the limit Z₀ >>L, Sis well preserved provided that rate of energy modification is "small"i.e., is adiabatic. This is conveniently expressed by the conditionαZ₀ >>1, where α is the average gain coefficient (averaged over thetotal fibre path). In the laser of FIG. 1, increasing the soliton pulseenergy by, say, a factor of 2 over a period of a few seconds(corresponding to a total fibre path length of ˜10⁶ km!), translates toan α ˜3×10⁻⁵ km⁻¹. For a typical Z₀ ˜0.7 km (t˜3 ps), adiabaticity isensured with Z₀ ˜2×10⁻⁵. A fundamental limitation to the 1/t.sup. 2scaling behaviour is the compression of the soliton period (Z₀proportional to t²). Eventually, as the pulse compresses, Z₀ becomescomparable to L and the pulse area suffers distortions. A simulation forL/Z=1.93 and 14 dB loss is depicted in FIG. 10(b) and shows theevolution of a soliton pulse together with a spreading radiativecomponent. FIG. 10(b) is a rather extreme case (ΔS˜0.3) but serves toillustrate the point that the pulse continually sheds energy and finallyevolves to a stable, somewhat broader soliton pulse. It is possible thatthis mechanism of pulse distortion is the origin of the low-levelpedestal observed in the experiments.

The model presented here is very successful in describing the observedcharacteristics of the erbium soliton laser. This is a consequence ofthe dominant role of nonlinearity and dispersion in the pulse formation.Our model is not intended to be a complete description and, strictlyspeaking, should follow pulse evolution form noise. Clearly, a key roleof the modulator 20 is to provide a seed modulation upon which thecompressive combination of SPM and dispersion can act. Broadly speaking,this evolution must take the form of a high-order soliton compression.In the steady state, although the modulator imposes an immeasureablefrequency chirp on the soliton pulses, it is highly likely that thephase modulation plays a major role in shaping the pulse pedestal i.e.in trapping the spreading radiative wave. We have also made no effect tomodel the more complex gain dynamics involved in the amplificationprocess. In fact, we find that in the long Z₀ limit, the soliton pulsesare insensitive to the exact details of the energy profile. Thesimulations presented here assume a constant value for D. We have alsomodelled a totally integrated soliton laser configuration, where avariety of dispersive components are combined. Here, for Z₀ >>L, therelevant dispersion is the average value over the amplification period.

The experimental and theoretical investigations strongly support theidea that the interplay between fibre nonlinearity and dispersion iscrucial to pulse formation. Indeed, the temporal, spectral and powercharacteristics of the laser output are appropriate for fundamentalsoliton pulses. From our theoretical studies, we find that in the limitZ₀ >L, the laser is well described by a simple NLSE model with E_(av)=E₁. In particular, the soliton energy, E, was observed to vary inaccordance with D/t. This scaling relationship is of particularrelevance to proposed applications. For example, in the case ofnonlinear all-optical switching, subpicosecond duration pulses may bedesirable in order to reduce the average power requirements from theswitching source. Since t is proportional to D for a given pulse energy,then low dispersion erbium fibre would be required for the shortestpulses. In addition, in order to minimise the pulse distortions incurredas the pulse compresses, a low loss, short resonator would be advisable(see FIG. 9). On the other hand, for soliton communication systems therequirement is for pulses with durations of some tens of picoseconds.This might be achieved by incorporating a lumped, highly dispersiveelement, e.g. a grating, in the cavity. In terms of our model, in thelong Z₀ limit, the soliton will adopt the average value of the cavitydispersion. Also, in order to eliminate pulse chirp a pure AM modulationscheme would be appropriate to initiate the mode-locking process.

FIG. 11 shows a ring laser incorporating a 13 m long Al₂ O₃ --GeO₂--SiO₂ host fibre 42 with an erbium doping level of 200-300 ppm, a coreradius of 2.5 μm and an index difference of 0.01. The laser 40 alsoincludes an integrated optic lithium niobate phase modulator 44 andoptical fibre couplers 46 and 48 fusion spliced together at the pointsmarked "X".

Fibre polarization controllers 50, 52 and 54 were incorporated into theFIG. 11 ring laser order to ensure correct polarization of the lightincident on the phase modulator 44.

The phase modulator 44 has a 3 GHz electrical bandwidth, an insertionloss of about 6 dB and a switching voltage of about 10 V. Gigahertzmodulation bandwidths together with low modulation voltages required forabout phase change make these devices potentially very attractive forthe mode locking of integrated fibre lasers.

Unlike the linear cavity laser described previously with reference toFIG. 1 where the mode-locker must be located close to an end mirror, ina ring laser there is no equivalent constraint on the position of themodulation. The choice of a ring configuration of FIG. 11 thereforegreatly facilitates the use of high speed, pigtail lithium niobatetechnology. A packaged/pigtailed GRINSCH (GRaded Index SeparateConfinement Hetrostructure) InGaAsP MQW semiconductor laser 55 withoutput pigtail 56 provides the pump source for the ring laser 40. Thethreshold current for the 1000 μm-long laser 55 was ˜25 mA. Its poweroutput increased linearly with current and at 450 mA output 36 mW with aspectral width (FWHM) of some 7 nm centred around 1.473 μm. The fibrepigtail 56 from the MQW device 55 is spliced to fibre coupler 48 whichpermits efficient coupling of the pump light into the erbium fibre 42.At the same time optical coupler 46 allowed the erbium emission to becoupled straight through to the coupler 48 which provided a 3 dB outputcoupling for the cavity.

In spite of the high cavity losses, laser action was achieved at a pumppower of only ˜15 mW at the input to the coupler 48. This thresholdpower was estimated from the 200 mA current typically applied to the MQWdevice 55. There are two counter-propagating light beams in the ringlaser 40 and, therefore, two possible outputs from the 3 dB coupler 46.As the drive current to the laser, 55 was increased, the CW output powerof both these outputs increased linearly. For the counter-clockwise beama CW output of ˜2 mW was typically recorded at a current of 450 mA. Theclockwise direction output beam was significantly lower in power (by ˜6dB) due to traversing the modulator immediately prior to outputcoupling.

Mode-locked operation was achieved by simply tuning the drive frequencyof the modulator 44 to a high harmonic of the fundamental cavityfrequency. We employed the amplified sinusoidal output (5-10 V_(rms)) ofa frequency synthesiser 58. The drive frequencies were restricted to <1GHz by the synthesiser/amplifier combination for the results reportedbelow. Since the total length of the fibre cavity was ˜13 m we inferthat ˜60 pulses were in the cavity at any one time. Typically,pedestal-free pulses were obtained with durations ˜3 ps and average peakpowers (counter-clockwise output) in the range 0.5-1.2 mW (0.2-0.5 W).In this ring configuration, the standard telecommunications fibreassociated with the couplers 46 and 48 and the pigtails of the modulator44 is not essential for the soliton shaping and could, in fact betotally non-soliton supporting because the erbium-doped waveguideproviding the gain for the laser 40 is chosen to provide the totalpositive group delay dispersion to ensure soliton pulse generation.

FIG. 12(a) shows an autocorrelation trace of the output of the ringlaser 40 recorded at an average power of 0.6 mW and a repetitionfrequency of 810.6 MHz. An autocorrelation function of a 2.8 ps (FWHM)sech² intensity is an excellent fit to the experimental data as isclearly shown by theoretical points depicted. The corresponding spectrumis shown in FIG. 12(b) from which a spectral width (FWHM) of 0.9 nm ismeasured. The time-bandwidth product (ΔtΔν) of 0.31 is also in excellentagreement with a sech² pulse shape.

The powers measured above are consistent with the values expected forfundamental soliton pulses. We note that in the "long" soliton period(Z₀) regime (i.e. Z₀ >>L) or the relevant soliton quantities (e.g.energy and dispersion) are simple averages over the amplificationperiod. For the 13 m length of erbium fibre 42, a low, positive groupdelay dispersion is expected. In addition, the ring configurationcontains approximately 2 m of standard telecommunications fibreassociated with the couplers 46, 48 and the pigtailed modulator 44. Ifwe take, for example, dispersion parameters D=+2 and +15 ps/nm/km forthe erbium fibre and standard fibre respectively, the average dispersioncan be calculated to be ˜3.7 ps/nm/km. For a 3 ps pulse, we calculatethe soliton period, Z₀ =950 m which easily satisfied the Z₀ >>Lcriterion. In addition, we calculate a soliton power, P_(i) ˜0.4 W(assuming A_(eff) =30 μm²). This is in good agreement with the measuredoutput powers of the counter-clockwise beam which in turn is areasonable estimate of the path average soliton power within the cavity.For the travelling wave modulator 44 used here, the counter-propagatinglight beams experience equivalent phase modulation depths provided thatthe drive frequencies are less than ˜1 GHz. Although the clockwiseoutput was significantly lower in power, the measured temporal andspectral characteristics were similar to the other arm. This is inkeeping with the fact that the path average power for the two directionswere essentially equal.

At higher pump powers, a slight compression of the output pulse wasobserved (to ˜2.4 ps) together with a low-level pedestal component (˜30ps FWHM). The corresponding spectral measurements revealed theappearance of a narrow (˜0.1 nm FWHM) spike close to the peak of thebroad (˜1 nm) soliton spectrum. In the configuration of FIG. 11,increasing the pump power did not produce the P_(i) proportional to 1/t²scaling previously noted with respect to the FIG. 1 embodiment. Weattribute this to a restriction of the laser bandwidth by residualetalon effects or birefringence in the lithium niobate phase modulator44. Consistent with this notion, we observed that for both CW and CWmode-locked operations, the inclusion of the modulator 44 gave rise to astrong wavelength discrimination in the output of the laser 40 which wasconfined to wavelengths separated by ˜0.7 nm.

What is claimed is:
 1. A laser including a soliton supporting waveguidedoped with a material which when pumped produces optical gain, saidwaveguide having a soliton period characteristic of said solitonsupporting waveguide, said soliton period being greater than anamplification period that is the round-trip fibre length of the laserwaveguide which provides optical gain.
 2. A laser as in claim 1 in whichthe laser comprises a mode-locked laser.
 3. A laser as in claim 1 or 2including a bandwidth limiting element connected in an optical cavity ofsaid laser.
 4. A laser as in claim 3 in which said bandwidth limitingelement comprises a lumped, highly dispersive element.
 5. A laser as inclaim 4 in which the highly dispersive element comprises a diffractiongrating.
 6. A laser as in claim 1 or 2 including a tuning filterconnected in an optical cavity of said laser.
 7. A laser as in claim 2,including means for driving said mode-locked laser at a pulse repetitionrate corresponding to a fundamental mode of an optical cavity of saidmode-locked laser.
 8. A laser as in claim 1 or 2 in which the waveguidecomprises an erbium doped silica based optical fibre.
 9. A laser asclaimed in claim 8 in which the optical fibre comprises SiO₂ --Al₂ O₃--P₂ O₅ with an erbium doping level of 1100 ppm, the fibre having a coreradius of 2.5 μm and a core-cladding refractive index difference of0.015.
 10. A laser as in claim 1 including means providing at most fivepulses propagating in the laser at any given time.
 11. A method forgenerating soliton pulses including the steps of generating solitonsusing a laser including a soliton supporting waveguide doped with amaterial which when pumped produces optical gain, said waveguide havinga soliton period characteristic of said soliton supporting waveguide,said soliton period being greater than an amplification period that isthe round-trip fibre length of said laser waveguide which providesoptical gain.
 12. A method as in claim 11, including the step ofmode-locking the laser.
 13. A method as in claim 11 or 12 includingbandwidth limiting pulse within an optical cavity of said laser.
 14. Amethod as in claim 13, wherein said bandwidth limiting is achieved usinga lumped, highly dispersive element.
 15. A method as in claim 14, inwhich the highly dispersive element utilizes a diffraction grating. 16.A method as in claim 11 or 12 including use of a tuning filter within anoptical cavity of said laser.
 17. A method as in claim 12 includingdriving said mode-locked laser at a pulse repetition rate correspondingto a fundamental mode of an optical cavity of said mode-locked laser.18. A method as in claim 11 or 12 in which the waveguide comprises anerbium doped silica based optical fibre.
 19. A method as in claim 18 inwhich the optical fibre comprises SiO₂ --Al₂ O₃ --P₂ O₅ with an erbiumdoping level of 1100 ppm, the fibre having a core radius of 2.5 μm and acore-cladding refractive index difference of 0.015.